s3:rpc_server/witness: add implementation based on CTDB_SRVID_IPREALLOCATED and ctdbd...

s3:rpc_server/witness: add implementation based on CTDB_SRVID_IPREALLOCATED and ctdbd_all_ip_foreach()

The design is relatively simple in the end:

- We use ctdbd_all_ip_foreach() in order to build an
  in memory list of interfaces(ip addresses) and
  record if:
  - they are currently available or not
  - if they node local or not

- The current list is would we use for the
  GetInterfaceList() call.

- Register[Ex] will create an in memory structure
  holding a queue for pending AsyncNotify requests.

- Unregister() will cancel pending AsyncNotify requests and
  let them return NOT_FOUND.

- CTDB_SRVID_IPREALLOCATED messages will cause we refresh
  with ctdbd_all_ip_foreach():
  - this will detect changes in the interface state
    and remove stale interfaces.
  - for each change the list of registrations is checked
    for a matching ip address and a RESOURCE_CHANGE
    will be scheduled in the queue of the registration,
    the started queue will trigger AsyncNotify responses

- We also register the connections with ctdb in order
  to give other nodes a chance to generate tickle-acks
  for the witness tcp connections.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>